The U.S. Supreme Court has ruled that the makers of generic drugs cannot be sued for failing to carry an adequate warning of side effects.

Generic drug makers are required to use the same labeling as their brand-name counterparts under federal regulations as interpreted by the Food and Drug Administration, Justice Clarence Thomas wrote in the majority opinion (PDF). Those federal regulations pre-empt state laws, he said.

The plaintiffs, Gladys Mensing and Julie Demahy, had developed tardive dyskinesia after taking the generic drug metoclopramide for their digestive problems.

The Supreme Court ruled two years ago in Wyeth v. Levine that makers of brand-name drugs can be sued for failure to warn. The decision was largely based on the companies’ ability to change their drug labels without FDA permission.

“We recognize that from the perspective of Mensing and Demahy, finding pre-emption here but not in Wyeth makes little sense,” Thomas wrote. “Had Mensing and Demahy taken Reglan, the brand-name drug prescribed by their doctors, Wyeth would control and their lawsuits would not be pre-empted. But because pharmacists, acting in full accord with state law, substituted generic metoclopramide instead, federal law pre-empts these lawsuits.”

“As a result of today’s decision, whether a consumer harmed by inadequate warnings can obtain relief turns solely on the happenstance of whether her pharmacist filled her prescription with a brand-name or generic drug,” Sotomayor wrote. “The court gets one thing right: This outcome ‘makes little sense.’ ”